The angels who did not keep their original position but forsook their own proper dwelling place, he has reserved with eternal bonds in dense darkness for the judgment of the great day. (Jude 1:6)

In Noah's day, the sons of God have relations with the daughters of men

Now when men started to grow in number on the surface of the ground and daughters were born to them, the sons of the true God began to notice that the daughters of men were beautiful. So they began taking as wives all whom they chose. Then Jehovah said: “My spirit will not tolerate man indefinitely, because he is only flesh. Accordingly, his days will amount to 120 years.” (Genesis 6:1-3)

The Nephilim were on the earth in those days and afterward. During that time the sons of the true God continued to have relations with the daughters of men, and these bore sons to them. They were the mighty ones of old times, the men of fame. (Genesis 6:4)

Consequently, Jehovah saw that man’s wickedness was great on the earth and that every inclination of the thoughts of his heart was only bad all the time. Jehovah regretted that he had made men on the earth, and his heart was saddened. So Jehovah said: “I am going to wipe men whom I have created off the surface of the ground, man together with domestic animals, creeping animals, and flying creatures of the heavens, for I regret that I have made them.” But Noah found favor in the eyes of Jehovah. (Genesis 6:5-8)